1. Field of the Invention
This invention relates to a process for etching a semiconductor device, and more particularly to a method for etching a contact for forming a contact hole having a sloped sidewall profile using a single process.
2. Discussion of the Related Art
As the integration of semiconductor elements advances, the size of each element becomes smaller and accordingly the size of a contact hole also becomes smaller. If a contact hole is formed with a dry etching method, the side walls are made vertical, creating an insufficient overlay margin at the time of carrying out a deposition process.
To solve these problems, an etching process for forming a contact hole having a sloped sidewall profile has been proposed.
FIGS. 1(a)-1(d) show conventional etching processes for forming a sloped contact hole utilizing multiple insulation films which have different etching rates from each other.
In FIG. 1(a), a photoresist layer 13 is coated and then patterned to form opening 14 on an insulation layer 12 on substrate 11. The insulation layer 12 includes a SiO.sub.2 layer 12A and a PSG layer 12B, which have different etching rates.
After patterning, opening 14 exposes a portion of the surface of the insulation layer 12. As shown in FIGS. 1(b) and 1(c), the exposed insulation layer 12 is etched under CHF.sub.3 /O.sub.2 gas chemistry with the photoresist pattern used as a mask.
The process for etching the insulation layer 12 is divided into two steps. First, an opening 15 is formed by plasma etching the PSG layer 12B of the insulation layer 12 until the lower SiO.sub.2 layer 12A is exposed with an O.sub.2 flow rate of about 15% of the total gas flow. Then, an opening 15' is formed by etching the SiO.sub.2 layer 12A until the substrate is exposed with an O.sub.2 flow rate of about 3% of the total gas flow.
The SiO.sub.2 layer 12A has a lower etching rate than the PSG layer 12B under CHF.sub.3. Since the PSG layer 12B is also etched at the time of etching the SiO.sub.2 layer 12A in the second step, the slope at the opening 15' is increased. During this step, O.sub.2 serves to reduce the polymer produced by the use of a polymer gas of CHF.sub.3.
Referring to FIG. 1(d), a contact hole having a predetermined slope can be formed by carrying out an etching process with an O.sub.2 flow rate of about 90%. In which case, the photoresist pattern 13 is etched faster than the insulation layer 14' and the PSG layer 12B is etched faster than the SiO.sub.2 layer 12A, thereby forming the contact hole having a predetermined slope.
The processes for forming a contact hole as shown in FIGS. 1(a)-1(d) requires three steps and includes etching the insulation layers of PSG and SiO.sub.2, which have different etching rates from each other.
In a first step, a small portion of a slope is formed by etching the PSG layer with 10-20% of the O.sub.2 flow of the total gas flow using CHF.sub.3 /O.sub.2 gas chemistry. In a second step, the SiO.sub.2 layer is etched with 1-8% of the O.sub.2 flow rate. In a third step the photoresist pattern together with the insulation layer is etched with 80-100% of the O.sub.2 flow rate, causing the PSG layer to have a greater slope.
A contact hole having a predetermined slope can be formed by etching the PSG and SiO.sub.2 insulation layers, which have different etching rates from each other, using several steps of controlled O.sub.2 flow.
The foregoing contact hole forming process is complicated because it requires three steps, and since the photoresist pattern is also etched at the time of etching the insulation layer, reproducibility of the process is poor.
FIGS. 2(a)-2(c) show conventional etching processes for forming a sloped contact hole utilizing both an isotropic etching and an anisotropic etching.
Referring to FIG. 2(a), by forming an interlayer insulation layer 22 on a first conductive layer 21, and by coating and patterning a photoresist film on the interlayer insulation layer 22, a photoresist pattern is formed.
An opening 24 is formed by carrying out an isotropic etching of the interlayer insulation layer 22 with the photoresist pattern 23 used as a mask, and a via hole is then enlarged downward under the opening 24 by carrying out an anisotropic etching of the interlayer insulation layer 22.
Referring to FIG. 2(b), the side walls and the upper surface of the photoresist pattern 23 are subjected to an isotropic etching as shown by the dotted lines 25, with a mask erosion step. By carrying out such isotropic etching steps and mask erosion steps repeatedly, a sloped via hole 26, as shown in FIG. 2(c), can be formed.
After forming the sloped via hole 26, photoresist pattern 23 is removed, and sloped in hole 26 is coated with a second conductive layer 27, as shown in FIG. 2(d).
The foregoing method for forming a contact hole is complicated because it requires repeating isotropic etching steps and mask erosion steps. Further, since the slope is stepped, it is not possible to form a contact hole having a smooth slope.
FIGS. 3(a)-3(d) show another conventional process for forming a sloped contact hole utilizing processes for etching a photoresist film and a substrate repeatedly.
As shown in FIG. 3(a), an opening 33 is formed by coating and patterning a photoresist film 32 on a substrate 31.
As shown in FIG. 3(b), the substrate 31 is subjected to an etching with the photoresist film 32 used as a mask, and then the polymer produced during the foregoing process is removed.
FIG. 3(c) shows etching a part of the photoresist film 32 in the dotted lines. Then, as shown in FIG. 3(d), the substrate 31 is again subjected to an etching with the photoresist film 32 used as a mask.
By repeating the foregoing processes, a sloped contact hole 34 can be completed. However, since the contact hole is formed by repeating the three steps of etching a photoresist film, etching a substrate process and removing a polymer, the foregoing process is complicated and provides poor reproducibility. Further, since the slope is stepped, a smooth contact hole with the desired slope can not be obtained.
FIGS. 4(a) and 4(b) show a conventional process for forming a sloped contact hole by controlling the flow of CHF.sub.3 /CF.sub.4.
Referring to FIG. 4(a), an insulation layer 42, such as an oxide film, is formed on a substrate (not shown).
The insulation layer 42 is formed thick enough to cover a metal conductive layer 41 already formed on the substrate.
A photoresist pattern 43 is then formed by coating and patterning a photoresist film on the insulation film 42.
FIG. 4(b) shows an exposed surface of the insulation layer 42 after etching through opening 44. Specifically, a contact hole 45 is formed by carrying out etching of the insulation layer 42 until the conductive layer 41 is exposed with the photoresist pattern 43 used as a mask under a CHF.sub.3 /CF.sub.4 gas chemistry mixture.
This etching process is carried out over two steps. First, a large slope is formed by etching an insulation layer 42 under a gas flow with 25% of CHF.sub.3 gas, and second a predetermined slope is formed by etching the insulation layer 42 again under a gas flow with 5-25% of CHF.sub.3 gas.
The foregoing etching processes are used for forming a contact hole having a predetermined slope by controlling the flow rate of CHF.sub.3 /CF.sub.4 gas. If the CF.sub.4 gas flow rate should become greater than the CHF.sub.3 flow rate, then it is not possible to form a contact hole having a predetermined slope due to formation of a polymer on the sidewalls of the contact hole during the first etching step.
Further, since the contact hole is formed through two processes requiring a controlled CHF.sub.3 /CF.sub.4 gas ratio, the foregoing processes are complicated and provide poor reproducibility.
As has been explained, the conventional etching processes for forming a sloped contact hole are complicated, provide for poor reproducibility, and also make it difficult to form a predetermined smooth slope due to those processes which only yield a stepped slope.